I propose a small “closed curcuit” unit, described below, which could be connected in a continuous pipe, to distill sea water in large quantities at low cost.Each unit, the size of a small row-boat, would be devided into two sections: the bioling section and the saved water section. A fresnel lens (or perhaps water filled bags to achieve the same) heat the dark-tinted water section, which is thermaly insulated from the rest of the sea water. The boiling steam is then passed into the clean “water storage”, where it condenses. The steam pipe is partially submerged in the sea, and so is the water storage, itself a long and thick pipe with an air snorkel, so that water can be extracted. The water storage has fins to let off the heat into the sea water. When water is being extracted from storage, the steam pipe automatically closes via a pneumatic-mechano contraption.

This distilled water could be pumped up to mountains and entered into the underground water system, turning to drink water, in over-pumped areas. Or it could be used for irrigation and for showering (for drinking you need minerals in the water otherwise you get cretin [edit: corrected from Creton] disease, brain dammage or skin boils – as what happened to residents of the Jewish portion of Jerusalem during the 1947 siege, who drank only boiled cistern water).

I got this halfdea after watching the funny GreenPowerScience.com movie on how simple and fast it is for the Ruhals to distill water from a mud-pond, using a large Fresnel lens to boil water in a bottle with solar energy, and just have it go into another bottle and condense.